tl:dr my partner said one of his closest friends, who is his ex although he refuses to confirm it to not out her, is very pretty and it bothers me

My (25F) partner (26M) “Joe” used to date “Kim” (26F) years ago. She was his first girlfriend, although the relationship was secret at the time because he was scared his parents, who didn’t allow him to date, would find out. Kim and Joe are still very close friends to this day. However, he has never confirmed he has dated her to anyone else to not out her, as he has said that his first ex is asexual and that she didn’t want anybody else to know. Over time, he has told me that his first ex was one of his closest friends before they dated and a transracial adoptee raised by a single mother - and the only person who fits this description is Kim. So I figured out pretty quickly who she was, but every time I ask him if they used to date, he says “no comment”.

Last weekend Joe, Kim and I were hanging out with a bunch of friends. After the hangout, Joe told me he found Kim very pretty. This bothered me, so I said “Do you still like your ex?” and he said “I never said anything to confirm or deny she was my ex!” This created a bit of tension. I don’t know how to deal with this, as he will never say explicitly that she is his ex. What should I do?